Wire drawing die



Dec. 27, 1966 E. L. KERN ETAL. 3,293,950

WIRE DRAWING DIE Filed Jan. 15, 1965 E/ec/n'c 0/ ,00 war Sou/C6 INVENTORS.

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HTTORNEY 3,293,950 WIRE DRAWING DIE Edward L. Kern and Dennis W. Hamill, Midland, Mich., assignors to Dow Corning Corporation, Midland, Mich., a corporation of Michigan Filed Jan. 15, 1965, Ser. No. 425,875 10 Claims. (Cl. 76-107) The present invention relates to wire drawing dies and more particularly to improved wire drawing dies and methods of making such devices.

Wire drawing dies must be formed of extremely hard materials in order to resist Wearing in the wire drawing process. If the die exhibits wear the dimension of the wire drawn through the die is changed. Thus, for dimensional control in the production of wire it has been common practice in the field to provide dies made of silicon carbide crystals or pressed silicon carbide, and for finishing, of diamond. The silicon carbide crystals are generally randomly grown in carbide furnaces. A crystal near the desired size is chosen and cut and ground to the desired die configuration. Diamond must also be cut and finished in a like manner. In other dies, formed and pressed silicon carbide is used to line the die. Since these materials are extremely hard, the cutting and finishing steps are difiicult and expensive. Furthermore, it has been found that many randomly produced silicon carbide crystals are not of the superior grade desirable for production of optimum dies, nor is the formed and pressed silicon carbide of such quality. A further problem in crystals is the fact that hardness is different in different crystal planes. Since it is virtually impossible to cut a die from the crystal in a manner to provide uniform hardness, uneven wear results.

Accordingly, the present invention has for an object the provision of a wire drawing die which is superior to those of the prior art.

A further object is to provide a method for producing a quality die which is cheaper than those methods used in prior art systems.

In accordance with these and other objects, there is provided by the present invention a method of pyrolytically growing silicon carbide in the general shape of the required die, by decomposing alkylsilanes, and then performing final machining on the deposited material. It has been found that dies so formed have advantages over prior art silicon carbide dies in hardness and mi formity and exhibit many economies over both prior art silicon carbide and diamond dies in the manufacturing process.

Other objects and attendant advantages of the present invention will become obvious by a consideration of the following detailed description when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a view in perspective of a rod formed of silicon carbide in accordance with the present invention prior to cutting, removal of the substrate, further deposition, and machining;

FIG. 2 is a view in perspective of the rod of FIG. 1 after removal of the substrate and illustrating longitudinal sectioning for further deposition of silicon carbide;

FIG. 3 is a somewhat diagrammatic view in partial cross-section illustrating the method of silicon carbide deposition;

FIG. -4 is a view in perspective of a solid silicon carbide rod before cutting and machining;

FIG. 5 is a finished wire drawing die made in accordance with the present invention; and

FIG. 6 is a view in cross section illustrating an alternate method of making dies in accordance with the present invention.

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Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the figures thereof, there is shown in FIG. 1 a tube 11 of silicon carbide deposited on a rod 12 of substrate material such as silicon. The silicon carbide is formed by reducing certain alkylsilanes with hydrogen at temperatures between about 1150 and 125 C. This has been accomplished in a reaction chamber using a total hydrogen flow up to 10 liters per minute made up of ratio "of a 20 to l-to-l to 1 H to dimethyldichlorosilane ratio. Other silanes such as trimethylmonochlorosilane, propyltrichlorosilane, and various mixtures including monomethyl and dimethyltrichlorosilanes may also be used. It has been found, however, that the ratio of starting carbon to starting silicon in the silanes must be at least 1.75 to 1. While a silicon rod has been found useful as a substrate, rods or tubes of other substrate materials capable of withstanding the temperatures involved, and capable of being easily removed from the deposited silicon carbide may also be used. The silicon carbide which was produced by this process has been found to have a Knoop g.) hardness value of 2960:30 kg./mm.

After a layer of silicon carbide which is sufiiciently thick to be easily workable has been deposited on the rod, the composite rod is removed from the reactor and a plurality of longitudinal cuts 13 (FIG. 2) are made to section the rod into pie shaped sectors. The substrate is removed by etching, sand blasting or any other suitable process leaving a plurality of truncated trapezoidal sectors 11a, 11b, 110, etc. The corners of these sectors are preferably then rounded by grinding and three of the .sectors are then formed into the two legs and the cross bridge of a reactor 14, as shown in FIG. 3, for further deposition.

The legs 11b and lie are connected to an electrical power source 15, and form a resistance heating element. Alkylsilane, as set forth hereinabove, and hydrogen are again flowed into the system through a nozzle 16 under the conditions heretofore enumerated. As is common in reaction chambers, an exhaust duct 21 is provided to remove gases from the reactor. The silane is again decomposed into silicon carbide which is deposited on the sectors 11a, 11b, and 110, thereby forming solid silicon carbide rods or cylinders. When the rods have reached sufiicient diameter for machining of the desired dies as shown by the numeral 17 in FIG. 4, they are removed from the reactor and machined by means such as diamond grinding tools on a lathe or by electron discharge machining. The rod is cut to the length desired for the dies and the inside portions are also machined to the desired final configuration such as that shown in FIG. 5.

By using a substrate which is easily preworked, such as tantalum tubing, it is possible to eliminate many of the grinding and cutting operations, as well as the double deposition steps. A consideration in choosing substrate material for this process, besides ductility and ability to withstand temperatures, is that the material must have a coefiicient of thermal expansion sufficiently close to that of the silicon carbide to prevent stresses sufficiently large to cause cracking of the silicon carbide. As shown in FIG. 6, a tubular substrate such as tantalum tube 18 is shaped on its outer surface to the desired configuration of a plurality of dies. Due to the ductile nature of tantalum and modern methods of forming tubing, this is easily accomplished. The tubing is resistance heated in the reactor and a silicon carbide layer 19 is deposited as hereinbefore described. In this method the deposition process is continued until the approximate size of the finished die has been reached. The composite tube is then cut at sec- 3 tions A -A B B and C -C to provide finished die lengths. The tantalum is easily removed from the silicon carbide by use of HFHNO leaving only final polishing and possible small amounts of machining necessary to complete the die.

Besides tubes and cylindrical rods, it is further possible to use as substrate, fine wires or rods having an exterior surface which has been shaped to conform to the desired die shape as was described with respect to tubing. Wires are particularly useful in making small dies. Suitable materials, other than silicon and tantalum include carbon, tungsten and various alloys.

As an aid to electron discharge machining it is possible during the deposition of the silicon carbide to dope the SiC to the optimum resistivity needed for conduction of current in this process. The Si-C can be doped to a resistivity as low as .05 ohmcms. using nitrogen, boron, phosphorous or other suitable dopants from doping sources such as N BCl PCl and others. The dopant is flowed into the reactor during decomposition, thereby providing uniform dopant distribution and a resultant simplifying of the machining process by using electron discharge machining.

The silicon carbide die made in accordance with the aforedescribed process and variations thereof are of harder material than silicon carbide dies formed by prior art processes, with the possible exception of some naturally grown crystals grown in silicon carbide furnaces. In comparison with these, however, dies made by the present process have the advantage of economy, as well as symmetry to provide even wear characteristics. It is also possible to produce larger dies than could be heretofore produced from natural crystals. A further advantage, particularly when a preformed substrate is used, is the lack of the extensive machining necessary to form natural crystals into dies. It is also possible with the present invention to dope the material to optimum resistivity for electron discharge machining, thereby simplifying the machining which is necessary with this invention.

Various other modifications and variations of the aforedescribed process will become obvious to those skilled in the art. It is to be understood, therefore, that within the scope of the appended claims the invention may be practiced, otherwise than as specifically described.

That which is claimed is:

1. A process for making dies comprising the steps of depositing from the vapor phase a layer of silicon carbide on a substrate,

removing the substrate, and

finishing said silicon carbide to the final die configuration.

2. A process for making dies as defined in claim 1 wherein said substrate is of a tubular configuration and said silicon carbide is deposited on the outer surface thereof.

3. A process for making dies as defined in claim 2 wherein said substrate is preformed to substantially the desired internal configuration of the die being made.

4. A process for making dies as defined in claim 1 wherein said substrate is preformed to substantially the desired internal configuration of the die being made.

5. A process for making dies as defined in claim 1 but further including the step of doping said silicon carbide to alter the electrical characteristics thereof.

6. A process for making dies as defined in claim 5 wherein said finishing step includes electron discharge machining.

7. A process for making dies as defined in claim 5 wherein said silicon carbide is deposited from vaporized alkylsilanes having a carbon to silicon ratio of at least 1.75 to 1.

8. A process for making dies as defined in claim 1 wherein said silicon carbide is deposited from vaporized alkylsilane having a carbon to silicon ratio of at least 1.75 to 1.

9. A process for making dies as defined in claim 1, but further including before the finishing step the steps of cutting said silicon carbide into longitudinal segments,

and

depositing from the vapor phase additional silicon carbide on said segments.

10. A process for making dies as defined in claim 9 wherein said silicon carbide is deposited from vaporized alkylsilane having a carbon to silicon ratio of at least 1.75 to 1.

No references cited.

GRANVILLE Y. CUSTER, JR., Primary Examiner. 

1. A PROCESS FOR MAKING DIES COMPRISING THE STEPS OF DEPOSITING FROM THE VAPOR PHASE A LAYER OF SILICON CARBIDE ON A SUBSTRATE, REMOVING THE SUBSTRATE, AND FINISHING SAID SILICON CARBIDE TO THE FINAL DIE CONFIGURATION. 